It likely was so dammed up it overflowed back down to the south, through the Aral, Caspian, and Black Seas, to the Mediterranean: https://folk.uib.no/ngljm/PDF_files/Mangerud_et_al_2004,_QSR...
I'm sure I remember reading a good article about it in National Geographic or Scientific American or something, but I can't find it.